This invention relates to signal-seeking, scanning radio receivers and, more particularly, relates to such receivers which are capable of receiving signals in a plurality of frequency bands and to such receivers including circuitry for tracking the operation thereof over the width of a frequency band.
Signal-seeking, scanning radio receivers are, of course, well known in the art; they have found a particularly useful application in receivers designed to receive signals at frequencies designated by the U.S. Federal Communications Commission for use in the public safety radio sevices. Examples of such receivers appear in U.S. Pat. Nos. 3,531,724 to G. H. Fathauer, 3,665,318 to S. J. Hoffman, et al., 3,714,585 to R. C. Koch, 3,821,651 to G. H. Fathauer, 3,794,925 to K. Imazeki, and in the copending application Ser. No. 303,016 filed Nov. 2, 1972 by G. H. Fathauer now U.S. Pat. No. 3,873,924. The frequencies now designated for use by the public safety radio services are commonly referred to as falling within four bands, the low or "L" band including frequencies within the range of approximately 30-50 MHz, the high or "H" band including frequencies within the range of approximately 150-174 MHz, the ultra-high frequency or "U" band including frequencies within the range of approximately 450-470 MHz, and the ultra-high frequency or "T" band including frequencies within the range of approximately 470-512 MHz. The scanning radio receivers of the prior art have generally not been operable on all four of those frequency bands. Moreover, of those scanning radio receivers of the prior art which were operable on three of those bands, many could only be operated on two of the four bands at the same time, i.e., such receivers generally required separate radio frequency sections for each of the separate bands but were only operable with two such radio frequency sections so that, in order to use the receiver on a third band, it was necessary to physically remove from the receiver the radio frequency section corresponding to one of the bands and replace it with a radio frequency section corresponding to the new band. Receivers which were simultaneously operable on more than two bands generally required an undesirably large number of components and were, correspondingly, undesirably expensive and, moreover, were physically undesirably large. Such receivers could, of course, be reduced in physical size but only at the expense of other design considerations. Moreover, such receivers by including separate radio frequency sections often had an undesirably large number of duplicate components in the separate radio frequency sections.
Further, many signal-seeking, scanning radio receivers of the prior art have had the disadvantage associated therewith that a compromise was required between the sensitivity of the receiver and the ability of the receiver to receive signals across entire ones of the frequency bands. Thus, the receiver radio frequency sections could be either relatively broadly tuned so that the receiver would receive signals across an entire band but at a relatively low sensitivity level or the radio frequency sections could be more narrowly tuned to increase the receiver sensitivity over a portion of the desired band but a corresponding decrease in the sensitivity over the remaining portions of the band would occur. Circuitry for tracking the tuning of a radio frequency section of a scanning radio receiver is shown in the above-identified copending application, but separate tracking circuitry is there shown for each band in which this feature is desired. It is desirable to use the same tracking circuitry for all bands of operation of the receiver. Further, it is desirable to improve the performance of the tracking circuitry shown in that application.